This invention relates to semiconductor injection lasers and, more particularly to heterostructure GaAs:GaAlAs lasers adapted for fundamental transverse mode operation.
A major concern in the development of semiconductor injection lasers has been to obtain lowest order transverse mode operation. This is because higher power outputs and improved linear light output versus pumping current input are being sought to meet requirements necessary, for example, for optical fiber transmission, optical disc writing and integrated optical components and circuits.
Heterostructure lasers are generally provided with a linear stripe geometry orthogonal to the cleaved ends of the device. Such linear or aligned stripe geometry may be an oxide stripe, diffused stripe, implanted stripe, substrate stripe or planar stripe. Lasers with geometry of this type experience nonlinearities in their light output for different pulsed operating conditions at power levels of only a few milliwatts. Also relaxation oscillations are observed during pulsed operation in the light output. These nonlinearities and oscillations are not desirable in achieving a high intensity and uniform output suitable for commercial applications.
Very narrow linear and orthogonal stripe configurations, such as, 2 .mu.m wide, lead to lowest order or fundamental transverse mode control near threshold, but at higher current levels, higher order modes are present even with such narrow stripe configurations.
It is generally thought that these output nonlinearities, often referred to as "kinks", are caused by several factors. First, the gain profile, during pumping, is characteristically bell shaped with maximum gain at its center. However, when several milliwatts of output power is achieved (for example 2 to 4 mW), gain saturation is reached. Residual gain at the tails of the profile approach or exceed that at the center of the profile. As a result, the gain profile changes, often having a double hump appearance and instablization of the transverse mode. In effect, recombination of injection carriers is occurring at a faster rate in the active region of the structure at a position directly below the center of width of the stripe relative to the edge or "wings" of the aligned stripe. Secondly, the fundamental mode, TE.sub.00, as well as other transverse modes, in particular, the TE.sub.01, mode, impinging on the facets of the laser couple back into their respective modes. In either case, mode nonstability persists.